The present invention relates to braze filler metal compositions which are iron-free. More specifically, the present invention relates to a braze filler metal which is particularly resistive to corrosion due to tap water sources containing chloride or fluoride ions and the like.
It has been desirable in the past to provide braze filler metals, for corrosion resistant metals such as stainless steel, which are resistant to oxidation and corrosion when utilized in a brazing situation. Thus, brazing filler metals have been successfully developed to resist particularly harsh environments such as sodium hydroxide solutions, sulfuric and nitric acid solutions and the like. Common among such brazing filler metals are nickel based alloys containing iron and chromium. An example of such a filler metal is the Wall Colmonoy Corporation NICROBRAZ.RTM. L.M. alloy which has a nominal composition including about 7.0% chromium; 3.0% iron; 3.1% boron; 4.5% silicon; and 0.06% carbon with the balance being nickel.
Some of these prior filler metals while being generally effective have been found to be susceptible to corrosion in tap water and particularly, when hot tap water was used for rinsing parts and the like. Many of these prior filler metals include significant iron content. It is believed that the iron content is problematic in leading to these corrosion problems because during brazing, iron from the base metal may diffuse into the molten braze fillet. After this diffusion the amount of chromium originally provided in the braze filler metal may not be enough to protect the new iron diffused into the braze fillet. Additionally, because the chromium in the braze filler metal and base metals may form chromium carbides during brazing, less chromium is available to protect the iron from corrosion. Thus, it has been a goal in the art to provide an iron-free braze filler metal.
Prior art braze filler metals showed corrosion susceptibility particularly when used at the upper acceptable temperature limits for such braze filler metals. This is believed to be attributable to the enhanced diffusion of iron into the braze metal at the higher temperatures. Additionally, with the lower quality stainless steels, such as 303, 304 and 316, grain enlargement of the base metal occurred during brazing. This is problematic in that grain enlargement will provide a pathway for the undesirable chromium carbide formation at the grain boundaries at the braze filler metal/base metal diffusion layer. Thus, chromium carbide formation at the grain boundaries is undesirable in that the result is again a net loss of chromium available for corrosion protection of the base metal and the braze filler metal.
Even in higher quality stainless steels iron will tend to diffuse into the braze filler metal. This will tend to render the braze metal more sensitive to corrosion. Also even in the higher quality stainless steels, chromium carbides may form in the grain boundaries of the base metal. This again reduces the amount of chromium which is available for corrosion protection of the base metal at the critical grain boundary area.
Thus, it has been a goal in the art to provide an iron-free filler metal composition which is not only corrosion resistant in its composition but will also act to protect the base metal from corrosion susceptibility caused by iron diffusion and chromium carbide formation.